IF THE AIR FILTER IS RESTRICTIVE THEN THAT MAKES THE FUEL MIXTURE RICH.
How to test the filter? Take it off and blow thru it while letting it seal around your mouth.
You shouldn't feel any additional pressure to blow thru it.
Change the oil ratio to 25:1
click on my signature link to find out how I leaned out my stock carb before tossing it in the trash and replacing it with a pro carb.

Here is a picture of my spark plug after I put the carb to the leanest mix and using 18:1.
So do I need to change the air/fuel mix? And am I ok with running my 18:1 fuel/oil mix for the rest of my tank?

use it up, or just add the same amount of fuel as what you have to it... modern oils are far superior to the muck they seem to use in china...

i run 50:1 with no internal issues yet... always been a lil pool of this red goop in the bottom of the crankcase when i pull em apart... ie, oil. and (until i painted over it the other day) its taken almost a year for this black streak from the exhaust join to wrap right round to the intake on one engine. flange is too thin . always warps a bit, not that that bothers me. if i keep filing it flat, it just gets thinner!

always get some smoke on first startup too. vanishes after a minute or so.

You can try running a little colder plug.
You don't mention where you live in your profile but if it's cold county you want a lower temp number in the spark plug size, this keeps more heat in the engine to prevent plug fouling.

For example I am in Phoenix, AZ where triple digit heat is a daily thing in summer so I run NGK BPR7HIX or even an 8 Iridium spark plugs to dissipate all the heat I can in all the 2-strokes I build, a 6 or even 5 may better for you.

Also when you have your plug out turn the fuel off and pedal the bike up and down the street with the clutch out to blow any excess fuel out of your crankcase, it's easy and worth it.